This discusses resources on open source contributing.
My initial approach was to clone the repo of the framework/library that I use daily and start building the code. If I have any build errors report them or try to improve the readme with why it's failing. Another thing will be to run the project tests against the latest version of the platform like run a Java project against JDK 10 and check for regressions that could be reported. You can try to fix and send a PR along with adding the target in CI config so that it's always tested. Then you can search the class that caused the error to see if other projects have the same problem.
Another area will be to port a module. I find porting a python module to clojure a good exercise when I was learning clojure. You can also try interfacing with a module like a wrapper from Clojure to a java library and so on. This is assuming you have a host platform on which the language runs.
I would also say open source is also more about docs, issues, testing etc. This article by Steve Klabnik was an inspiring read.
You can also check out below for good first issues to work on.